U.S. patent application number 16/429986 was filed with the patent office on 2019-12-05 for magnetic elements and methods for the additive manufacture thereof.
This patent application is currently assigned to California Institute of Technology. The applicant listed for this patent is California Institute of Technology. Invention is credited to John Paul C. Borgonia, Ryan W. Conversano, Robert P. Dillon, Samad A. Firdosy, Adam Herrmann, Bryan W. McEnerney, Andrew A. Shapiro-Scharlotta.
Application Number | 20190366435 16/429986 |
Document ID | / |
Family ID | 68695321 |
Filed Date | 2019-12-05 |
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United States Patent
Application |
20190366435 |
Kind Code |
A1 |
Firdosy; Samad A. ; et
al. |
December 5, 2019 |
Magnetic Elements and Methods for the Additive Manufacture
Thereof
Abstract
Elements formed from magnetic materials and their methods of
manufacture are presented. Magnetic materials include a magnetic
alloy material, such as, for example, an Fe--Co alloy material
(e.g., the Fe--Co--V alloy Hiperco-50.RTM.). The magnetic alloy
materials may comprise a powdered material suitable for use in
additive manufacturing techniques, such as, for example direct
energy deposition or laser powder bed fusion. Manufacturing
techniques include the use of variable deposition time and energy
to control the magnetic and structural properties of the materials
by altering the microstructure and residual stresses within the
material. Manufacturing techniques also include post deposition
processing, such as, for example, machining and heat treating. Heat
treating may include a multi-step process during which the material
is heated, held and then cooled in a series of controlled steps
such that a specific history of stored internal energy is created
within the material. Magnetic elements may include, for example,
motors, generators, solenoids and switches, sensors, transformers,
and hall thrusters, among other elements.
Inventors: |
Firdosy; Samad A.; (La
Crescenta, CA) ; Dillon; Robert P.; (Long Beach,
CA) ; Conversano; Ryan W.; (Tarzana, CA) ;
Borgonia; John Paul C.; (Santa Fe Springs, CA) ;
Shapiro-Scharlotta; Andrew A.; (Glendale, CA) ;
McEnerney; Bryan W.; (Redondo Beach, CA) ; Herrmann;
Adam; (Piqua, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
California Institute of Technology |
Pasadena |
CA |
US |
|
|
Assignee: |
California Institute of
Technology
Pasadena
CA
|
Family ID: |
68695321 |
Appl. No.: |
16/429986 |
Filed: |
June 3, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62679358 |
Jun 1, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01F 41/0206 20130101;
B22F 2003/248 20130101; C21D 6/004 20130101; B22F 2998/10 20130101;
C22C 33/0207 20130101; C21D 2251/00 20130101; B33Y 80/00 20141201;
B22F 2999/00 20130101; B22F 2998/10 20130101; B22F 3/1055 20130101;
B22F 2207/01 20130101; B22F 2003/248 20130101; B22F 3/1055
20130101; C21D 6/007 20130101; C21D 1/26 20130101; C22C 2202/02
20130101; B22F 3/1055 20130101; C22C 38/10 20130101; B33Y 10/00
20141201; C21D 9/0068 20130101; B22F 2999/00 20130101; C21D 9/40
20130101; C21D 1/74 20130101; C21D 1/773 20130101; C21D 9/0075
20130101 |
International
Class: |
B22F 3/105 20060101
B22F003/105; C22C 38/10 20060101 C22C038/10; C22C 33/02 20060101
C22C033/02 |
Goverment Interests
STATEMENT OF FEDERAL SUPPORT
[0002] The invention described herein was made in the performance
of work under a NASA contract NNN12AA01C, and is subject to the
provisions of Public Law 96-517 (35 USC 202) in which the
Contractor has elected to retain title.
Claims
1. A method for forming an element having magnetic properties
comprising: providing a powderized feedstock of a magnetic alloy;
additively manufacturing a magnetic element having a density of at
least 98% from a plurality of successive layers of deposited
powderized feedstock using a thermal energy source having a
specific energy of deposition between about 30 and 60 J/mm.sup.2;
and heat treating the magnetic element comprising: annealing the
magnetic element in a vacuum atmosphere at a temperature between
about 830.degree. C. and 875.degree. C. for at least 4 hours.+-.10
minutes, quenching the annealed magnetic element to at least below
500.degree. C..+-.20.degree. C. at a cooling rate of at least
80.degree. C./hour.+-.10.degree. C./hour in a vacuum atmosphere,
and cooling the quenched magnetic element to at least below a
temperature of 100.degree. C..+-.20.degree. C. in an atmosphere
having a concentration of oxygen less than about 10 ppm; wherein
the heating and cooling of the magnetic element during the additive
manufacturing and heat treating creates an internal energy that at
least partially determines the grain structure of the magnetic
alloy of the magnetic element.
2. The method of claim 1, wherein the magnetic alloy is a Fe--Co
alloy.
3. The method of claim 2, wherein the magnetic alloy further
comprises V.
4. The method of claim 3, wherein the magnetic alloy is
Hiperco.RTM. 50.
5. The method of claim 1, wherein the vacuum atmosphere has a
pressure below 1.times.10.sup.-4 Torr.
6. The method of claim 1, wherein the minimum temperature is at
least 865.degree. C..+-.20.degree. C.
7. The method of claim 1, wherein the cooling is conducted in a
partial atmosphere of an inert gas.
8. The method of claim 1, wherein the additive manufacturing
process is selected from the group of direct energy deposition
(DED) or powder bed fusion (PBF).
9. The method of claim 1, wherein the thermal energy source is a
laser.
10. The method of claim 1, further comprising modifying the
internal energy of the magnetic alloy prior to the heat treating
such that a desired grain structure will be formed in the magnetic
alloy of the heat treated magnetic element.
11. The method of claim 10, wherein the internal energy is modified
by controlling a thermal energy source parameter selected from the
group of spot size, raster speed, and power.
12. The method of claim 10, wherein the grain structure of the
magnetic alloy is varied within the magnetic element.
13. The method of claim 12, wherein the grain structure varies
along a radial direction of the magnetic element.
14. The method of claim 12, wherein the variance of the grain
structure leads to variance in one or both of a magnetic property
of the magnetic alloy, selected from the group of coercivity,
permeability, magnetic saturation, core loss and flux density; and
a mechanical property of the magnetic alloy selected from the group
of hardness and strength.
15. The method of claim 1, wherein the method further comprises:
providing at least one additional feedstock alloy; and varying the
composition of the alloy being additively manufactured between at
least the one additional feedstock alloy and the magnetic alloy
such that the magnetic element has at least a first portion formed
from the magnetic alloy, at least a second portion formed from the
at least one additional alloy, and at least a third portion formed
by compositional gradients therebetween.
16. The method of claim 15, wherein the magnetic alloy is an Fe--Co
alloy and the at least one additional alloy is selected from the
group of stainless steel alloys and ferrous metal alloys.
17. The method of claim 15, wherein there are at least two
additional feedstock alloys.
18. The method of claim 1, wherein the magnetic element is selected
from the group of: motors, generators, solenoids and switches,
sensors, transformers, and Hall thrusters.
19. A magnetic element comprising: a body formed from a plurality
of successive additively bonded layers of at least a magnetic alloy
comprising at least Fe and Co, wherein the body has a density of at
least 98%, and a maximum magnetic flux density of at least 2 A/m, a
coercivity of less than 1.1 Oe, and a permeability of greater than
7000 H/m.
20. The magnetic element of claim 19, wherein the body further
comprises a plurality of successive additively bonded layers of at
least one additional alloy, such that the body has at least a first
portion formed from the magnetic alloy, at least a second portion
formed from the at least one additional alloy, and at least a third
portion formed by compositional gradients therebetween.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 62/679,358, filed Jun. 1, 2018, the disclosure of
which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The current disclosure is directed generally to elements
having \ magnetic properties and methods for their additive
manufacture.
BACKGROUND OF THE INVENTION
[0004] Soft magnetic alloys (e.g., Fe--Co--V alloys such as
Hiperco.RTM. 50) are commonly used for magnetic shielding in
elements such as Hall thrusters. These materials are also used
within industry to increase functionality, margin, and/or
reliability of motors, generators, solenoids and switches, sensors,
transformers, and magnetic disk read heads, etc. This is because
these soft magnetic alloy materials have high magnetic saturation,
high permeability, low coercivity, and low core loss.
Unfortunately, while such alloys demonstrate excellent magnetic
properties, they also tend to be brittle, making it difficult to
machine these materials. Moreover, the hallmark magnetic properties
of these alloys are subject to degradation during manufacture and
subsequent processing. For example, groups studying such materials
have reported the strong deterioration of magnetic properties of
ordered intermetallic Fe--Co--V alloys when they are subjected to
mechanical strengthening. (See, e.g., Hug, et al., J. Magnetism
& Magnetic Materials, 215-216 (2000) 197-200, the disclosure of
which is incorporated herein by reference.)
BRIEF SUMMARY OF THE INVENTION
[0005] The application is directed to elements having magnetic
properties and methods for their additive manufacture.
[0006] Many embodiments are directed to methods for forming an
element having magnetic properties including: [0007] providing a
powderized feedstock of a magnetic alloy; [0008] additively
manufacturing a magnetic element having a density of at least 98%
from a plurality of successive layers of deposited powderized
feedstock using a thermal energy source having a specific energy of
deposition between about 30 and 60 J/mm.sup.2; and [0009] heat
treating the magnetic element comprising: [0010] annealing the
magnetic element in a vacuum atmosphere at a temperature between
about 830.degree. C. and 875.degree. C. for at least 4 hours.+-.10
minutes, [0011] quenching the annealed magnetic element to at least
below 500.degree. C..+-.20.degree. C. at a cooling rate of at least
80.degree. C./hour.+-.10.degree. C./hour in a vacuum atmosphere,
and [0012] cooling the quenched magnetic element to at least below
a temperature of 100.degree. C..+-.20.degree. C. in an atmosphere
having a concentration of oxygen less than about 10 ppm; [0013]
wherein the heating and cooling of the magnetic element during the
additive manufacturing and heat treating creates and changes the
internal energy that at least partially determines the grain
structure of the magnetic alloy of the magnetic element.
[0014] In still many embodiments, the magnetic alloy is a Fe--Co
alloy.
[0015] In yet many embodiments, the magnetic alloy further
comprises V.
[0016] In still yet many embodiments, the magnetic alloy is
Hiperco.RTM. 50.
[0017] In still yet many embodiments, wherein the vacuum atmosphere
has a pressure below 1.times.10.sup.-4 Torr.
[0018] In still yet many embodiments, the minimum temperature is at
least 865.degree. C..+-.20.degree. C.
[0019] In still yet many embodiments, the cooling is conducted in a
partial atmosphere of an inert gas.
[0020] In still yet many embodiments, the additive manufacturing
process is selected from the group of direct energy deposition
(DED) or powder bed fusion (PBF).
[0021] In still yet many embodiments, the thermal energy source is
a laser.
[0022] In still yet many embodiments, the method further includes
modifying the internal energy of the magnetic alloy prior to the
heat treating such that a desired grain structure will be formed in
the magnetic alloy of the heat treated magnetic element.
[0023] In still yet many embodiments, the internal energy is
modified by controlling a thermal energy source parameter selected
from the group of spot size, raster speed, and power.
[0024] In still yet many embodiments, the grain structure of the
magnetic alloy is varied within the magnetic element.
[0025] In still yet many embodiments, the grain structure varies
along a radial direction of the magnetic element.
[0026] In still yet many embodiments, the variance of the grain
structure leads to variance in one or both of a magnetic property
of the magnetic alloy, selected from the group of coercivity,
permeability, magnetic saturation, core loss and flux density; and
a mechanical property of the magnetic alloy selected from the group
of hardness and strength.
[0027] In still yet many embodiments, the method further comprises:
[0028] providing at least one additional feedstock alloy; and
[0029] varying the composition of the alloy being additively
manufactured between at least the one additional feedstock alloy
and the magnetic alloy such that the magnetic element has at least
a first portion formed from the magnetic alloy, at least a second
portion formed from the at least one additional alloy, and at least
a third portion formed by compositional gradients therebetween.
[0030] In still yet many embodiments, the magnetic alloy is an
Fe--Co alloy and the at least one additional alloy is selected from
the group of stainless steel alloys and ferrous metal alloys.
[0031] In still yet many embodiments, there are at least two
additional feedstock alloys.
[0032] In still yet many embodiments, the magnetic element is
selected from the group of: motors, generators, solenoids and
switches, sensors, transformers, and Hall thrusters.
[0033] Various embodiments are directed to magnetic elements
including: [0034] a body formed from a plurality of successive
additively bonded layers of at least a magnetic alloy comprising at
least Fe and Co, wherein the body has a density of at least 98%,
and a maximum magnetic flux density of at least 2 Nm, a coercivity
of less than 1.1 Oe, and a permeability of greater than 7000
H/m.
[0035] In still various embodiments, the body further comprises a
plurality of successive additively bonded layers of at least one
additional alloy, such that the body has at least a first portion
formed from the magnetic alloy, at least a second portion formed
from the at least one additional alloy, and at least a third
portion formed by compositional gradients therebetween.
[0036] Additional embodiments and features are set forth in part in
the description that follows, and in part will become apparent to
those skilled in the art upon examination of the specification or
may be learned by the practice of the disclosure. A further
understanding of the nature and advantages of the present
disclosure may be realized by reference to the remaining portions
of the specification and the drawings, which forms a part of this
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] The description will be more fully understood with reference
to the following figures, which are presented as exemplary
embodiments of the invention and should not be construed as a
complete recitation of the scope of the invention, wherein:
[0038] FIG. 1 provides a flow-chart of a process for additively
manufacturing magnetic elements in accordance with embodiments of
the invention.
[0039] FIG. 2 provides a data graph of the equilibrium
thermodynamic phase diagram of Fe--Co materials in accordance with
the prior art.
[0040] FIG. 3 provides a flow-chart of a heat-treatment process for
additively manufacturing magnetic elements in accordance with
embodiments of the invention.
[0041] FIGS. 4a and 4b provide SEM images of a magnetic element
formed in accordance with embodiments of the invention.
[0042] FIG. 5 provides a data graph of the magnetic flux of a
magnetic element formed in accordance with embodiments of the
invention.
[0043] FIG. 6 provides a data graph of the Vickers hardness of a
magnetic element in accordance with embodiments of the
invention.
[0044] FIG. 7a provides a pair of data graphs of the magnetic flux
of a magnetic element in accordance with embodiments of the
invention.
[0045] FIG. 7b provides a table summarizing the data graph of FIG.
7a.
[0046] FIG. 8a provides an image of a gradient magnetic element in
accordance with embodiments of the invention.
[0047] FIGS. 8b and 8c provide a SEM images of a gradient magnetic
element in accordance with embodiments of the invention.
[0048] FIG. 8d provides a data graph of the microhardness of a
gradient magnetic element in accordance with embodiments of the
invention.
[0049] FIG. 8e provides micrograph images of a gradient magnetic
element in accordance with embodiments of the invention.
[0050] FIG. 9a provides diagrams of exemplary Hall thruster
elements capable of being manufacture in accordance with
embodiments of the invention.
[0051] FIG. 9b provides images of as-printed and annealed grain
structures of elements additively manufactured in accordance with
varying parameters in accordance with embodiments of the
invention.
[0052] FIG. 9c provides SEM images of annealed grain structures of
elements additively manufactured in accordance with varying
parameters in accordance with embodiments of the invention.
[0053] FIG. 9d provides a data graph of a VSM study conducted on
as-printed and annealed Hiperco.RTM. 50 along the build direction
and perpendicular to the build direction in accordance with
embodiments of the invention.
[0054] FIG. 9e provides an image of exemplary magnetic rings formed
from Hiperco.RTM. 50 in accordance with embodiments of the
invention.
[0055] FIG. 9f provides grain structure of the exemplary magnetic
rings of FIG. 9e.
[0056] FIG. 9g provides top surface views of the exemplary magnetic
rings of FIG. 9e.
[0057] FIG. 9h provides an image of exemplary magnetic rings formed
from Fe--Co in accordance with embodiments of the invention.
[0058] FIG. 9i provides grain structure of the exemplary magnetic
rings of FIG. 9h.
[0059] FIG. 9j provides top surface views of the exemplary magnetic
rings of FIG. 9h.
[0060] FIGS. 9k and 9l provide images of powder cross-section and
ring section for: (9k) Hiperco.RTM. 50, and (9l) FeCo in accordance
with embodiments of the invention.
[0061] FIG. 9m provides a data graph of magnetic flux densities for
Hiperco.RTM. 50 magnetic rings in accordance with embodiments of
the invention.
[0062] FIG. 10a provides a micrograph image of an element formed
with an energy gradient deposition in accordance with embodiments
of the invention.
[0063] FIGS. 10b and 10c provide micrograph images of a region of
the element of FIG. 10a formed with a high (10b) and low (10c)
energy density deposition in accordance with embodiments of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0064] Turning now to the drawings, elements formed from magnetic
materials and their methods of manufacture are described. In
various embodiments, the magnetic materials comprise a magnetic
alloy material, such as, for example, an Fe--Co alloy material
(e.g., the Fe--Co--V alloy Hiperco-50.RTM.). In many embodiments,
the magnetic alloy materials may comprise a powdered material
suitable for use in additive manufacturing techniques, such as, for
example direct energy deposition or laser powder bed fusion.
Various embodiments are also directed to methods of manufacturing
elements formed from magnetic materials. Many embodiments of
manufacturing techniques comprise the use of variable deposition
time and energy to control the magnetic and structural properties
of the materials by altering the microstructure and residual
stresses within the material. Manufacturing techniques according to
some embodiments include post deposition processing, such as, for
example, machining and heat treating. Heat treating embodiments may
include a multi-step process during which the material is heated,
held and then cooled in a series of controlled steps such that a
specific history of stored internal energy is created within the
material. In various embodiments, the magnetic elements formed
according to methods have magnetic properties that exceed the
state-of-the-art for similarly machined parts. Magnetic properties
(e.g., coercivity) and mechanical properties (e.g., hardness) of
elements can be tailored using additive manufacturing and heat
treatment to architect final microstructure according to
embodiments. It will be understood that magnetic elements according
to embodiments may include, for example, motors, generators,
solenoids and switches, sensors, transformers, and Hall thrusters,
among other elements.
[0065] The described apparatuses, systems, and methods should not
be construed as limiting in any way. Instead, the present
disclosure is directed toward all novel and nonobvious features and
aspects of the various disclosed embodiments, alone and in various
combinations and sub-combinations with one another. The disclosed
methods, systems, and apparatus are not limited to any specific
aspect, feature, or combination thereof, nor do the disclosed
methods, systems, and apparatus require that any one or more
specific advantages be present or problems be solved.
[0066] Although the operations of some of the disclosed methods are
described in a particular, sequential order for convenient
presentation, it should be understood that this manner of
description encompasses rearrangement, unless a particular ordering
is required by specific language set forth below. For example,
operations described sequentially may in some cases be rearranged
or performed concurrently. Moreover, for the sake of simplicity,
the attached figures may not show the various ways in which the
disclosed methods, systems, and apparatuses can be used in
conjunction with other systems, methods, and apparatus.
[0067] State-of-the-art soft magnetic alloys such as the Fe--Co--V
alloy Hiperco.RTM. 50 are used for magnetically shielding
applications, such as in motors, generators, solenoids and
switches, sensors, transformers, magnetic disk read heads, and Hall
thrusters. These materials are also used by industry to increase
functionality, margin, and/or reliability of these elements. This
is because these materials generally have high magnetic saturation,
high permeability, low coercivity, and low core loss.
Unfortunately, these state-of-the-art materials also tend to be
brittle, making it difficult to machine these materials. What is
more, the magnetic properties are very sensitive to how they are
processed, and can be significantly degraded during such
processing.
[0068] To address the issues that arise in working with these
materials during manufacture, some groups have attempted to
manufacture parts from such magnetic alloys via additive
manufacturing (AM), or 3D printing, from powder using direct energy
deposition or similar laser based AM process. However, these early
attempts have met with mixed results. Although elements having
near-net shape were formed, the magnetic properties of elements
formed using such techniques was determined to be sub-optimal.
(See, e.g., Kustas, et al., Additive Manufacturing, 21 (2018)
41-52, the disclosure of which is incorporated herein by
reference.)
[0069] Embodiments of the instant disclosure are directed to
methods for implementing additive manufacturing techniques to
enable near-net fabrication of difficult to machine
state-of-the-art magnetic materials for elements such as, for
example, motors, generators, solenoids and switches, sensors,
transformers, and Hall thrusters having excellent mechanical and
magnetic properties that are tunable for specific implementations,
and the magnetic elements formed thereby.
Embodiments Implementing Methods of Manufacturing Soft Magnetic
Elements
[0070] Turning to the drawings, many embodiments are directed to
methods of manufacturing elements with magnetic properties via an
additive manufacturing process. As shown in FIG. 1, in many
embodiments the process comprises the general steps of preparing a
feedstock of a powderized magnetic alloy material, depositing the
powderized magnetic material through a suitable additive
manufacturing process to form a magnetic element, and
post-processing the formed magnetic element through at least a heat
treatment process.
[0071] In many embodiments, the feedstock comprises an alloy having
magnetic properties. Although any suitable magnetic metal alloy
(e.g., an alloy having a very high maximum magnetic saturation and
very low magnetic coercivity) capable of being powderized and
deposited using an additive manufacturing process may be used, in
many embodiments the alloy comprises at least Fe and Co as
represented by the binary equilibrium phase diagram summarized in
FIG. 2. These Fe--Co alloys show very high magnetic permeability,
low coercivity, high saturation and high Curie temperature.
Although Fe--Co alloys have excellent magnetic properties, they are
notoriously brittle and difficult to machine, which limits their
usefulness. Accordingly, conventionally Fe--Co alloys are alloyed
with one or more materials to improve manufacturability, such as,
for example, V. One exemplary Fe--Co--V alloy is Hiperco.RTM. 50
manufactured by Carpenter Powder Products, the composition of which
is given in Table 1, below. Despite the widespread use of such
trenary and higher-order alloys, the improved
formability/machinability comes at the expense of the magnetic,
electrical and thermal properties of the base Fe--Co binary alloy.
In short, the binary Fe--Co alloy has better magnetic, electrical
and thermal properties as compared to Hiperco.RTM. 50 (Fe-49Co-2V),
but is not used due its very poor formability/machinability.
Vanadium is added to improve the mechanical properties and poor
formability/machinability at the expense of magnetic, electrical
and thermal properties. Implementing additive manufacturing to form
near net shape magnetic elements, according to embodiments, allows
for the use of higher magnetic performance Fe--Co materials.
Accordingly, embodiments of the current disclosure advantageously
may be used with any suitable magnetic alloy, including Fe--Co--V
alloys as well as the conventionally unavailable Fe--Co alloys.
TABLE-US-00001 TABLE 1 Composition of Carpenter Mirco-Melt .RTM.
Hiperco .RTM. 50 Fe Co V Si Ni Cr Mn P Mo Al C S O N BAL 48.79 1.76
0.01 0.04 0.01 0.02 0.004 0.01 0.001 0.005 0.002 0.0235 0.003
[0072] Various additive manufacturing techniques suitable for
forming a bulk object, layer-by-layer, at near net shape to
finished dimensions, using a magnetic metal alloy materials may be
used. In various embodiments, the process may be an energetic
emission (e.g., laser, e-beam, etc.) based process, such as, for
example direct energy deposition (DED) or powder bed fusion (PBF).
DED and PDF are additive manufacturing processes in which focused
thermal energy is used to fuse materials. While lasers may be the
most popular "focused thermal energy" source for these processes,
electron beam and plasma arc systems may also be used in accordance
with embodiments.
[0073] Although both DED and PBF use an energetic energy source to
heat a feedstock, there are some differences. In powder-fed DED,
the material being fused is deposited by "blowing" metallic powder
through small nozzles or orifices into a melt pool created by the
energetic source (e.g., laser). Depending on the power and type of
energetic source being used, the energetic source beam is focused
to create a known spot size (for example, in a laser-based system
the spot size may be 0.5 or 1 mm for a 500-watt laser versus 1.5 or
3 mm for a 2.5-kilowatt laser). The depth and velocity of the
resulting melt pool are dictated by the scan speed of the laser (or
movement of the part on the build platform underneath the laser),
and the energy absorption and thermal conductivity of the feedstock
that is being deposited. The size of the melt pool, the speed at
which the laser moves and the powder feed rate (the rate at which
powder is blown through the nozzles toward the laser) dictate how
much powder is captured in the melt pool and ultimately how much
material is fused to the part--the layer beneath as well as
adjacent material within the layer currently being built. A large,
hot, slow-moving melt pool will have a higher powder capture rate
(70% to 80%, which is the best case) than a smaller or faster
moving melt pool (20% to 30% capture rate, which is the worst
case). However, the thermal history and therefore the
microstructure and mechanical properties of the part will be
different in the two cases. This is one of the challenges with DED,
namely, tuning process parameters to ensure a fast build time,
efficient use of powder, and a part that is dimensionally accurate.
The main difference with PBF processes is that while in DED
material is only deposited where needed (e.g., within the melt pool
created by the energetic emission source), in PBF each layer
requires a new layer of powder to be spread. This generally makes
PBF systems slower, but in exchange there is generally better
resolution and surface finish.
[0074] Regardless of the specific additive manufacturing technique
used, it will be understood that by adjusting process parameters
(e.g., energy density of laser energy or raster time), the
as-printed microstructure of a formed element may be altered to
allow the formation of larger or smaller grains after
post-processing annealing. In various embodiments, the thermal
history of the formed element may be controlled by altering the
spot size, raster speed, power, and laser movement during
manufacture. Various embodiments have a specific energy of
deposition (e.g., laser energy) of between about 30 and 60
J/mm.sup.2 such that full density additively manufactured elements
(e.g., at least greater than .about.98%, which is required under
ASTM A773 for DC magnetic elements) are formed. Test data showing
energy density requirements to form fully dense elements according
to some embodiments are provided in Table 2, below.
TABLE-US-00002 TABLE 2 Additive Manufacturing Processing Parameters
Laser Power Build Velocity Specific Energy Density (W) (mm/s)
(J/mm.sup.2) (%) 650 16.9 38 99 .+-. 1.0 1070 16.9 36 99 .+-. 1.0
1680 16.9 46 99 .+-. 1.0
[0075] Turning to the post-process heat treatment, as shown in the
flow-chart provided in FIG. 3, the process generally comprises at
least three steps: [0076] annealing of the solidified element at a
heat treatment temperature for a set period of time to allow the
grain structure to stabilize in a desired configuration; [0077] an
initial fast cooling or quenching process to bring the temperature
of the element below an initial threshold temperature; and [0078] a
second slower cooling process to bring the temperature of the
element below a second threshold temperature.
[0079] In various embodiments the annealing step requires the
formed element to be held at at least 830.degree. C. but without
exceeding about 875.degree. C. for at least 4 hours.+-.10 minutes.
In other embodiments the minimum temperature is 865.degree.
C..+-.20.degree. C. In many embodiments the annealing is conducted
in a vacuum. In many such embodiments the vacuum atmosphere has a
maximum pressure of 1.times.10.sup.4 Torr. In many embodiments the
initial quenching process involves cooling the annealed element
from the annealing temperature to at least below 500.degree.
C..+-.20.degree. C. at a cooling rate of at least 80.degree.
C./hour.+-.10.degree. C./hour. In many embodiments the second
cooling step involves air cooling the quenched element to at least
below a temperature of 100.degree. C..+-.20.degree. C. In many such
embodiments the air cooling is conducted in a partial atmosphere of
inert gas, such as, for example, argon where the concentration of
oxygen in the atmosphere is preferably less than 10 ppm. The
importance of a suitable heat-treatment process on the properties
of the elements formed in accordance with embodiments is summarized
in the Exemplary Embodiments, below.
[0080] Although specific heat treatment parameters are described
above, it will be understood that the annealing and quenching steps
may be modified to ensure a desired grain structure is achieved in
the as-formed element, as will be described in greater detail
below. It will be also understood that other post-process machining
steps may be included in the process including, for example,
grinding, sanding, buffing, drilling, cutting, etc.
[0081] During the additive manufacturing deposition (e.g., DED,
PBF, etc), the process involves melting the target metal alloy via
heating (e.g., by a laser emission), then solidifying the material
via cooling, and then heat treating (e.g., annealing) the formed
sample in a post-processing step. These heating and cooling cycles
create an energy history and stored internal energy in the formed
element. Generally, the faster the element cools, the higher the
stored internal energy in the element. This internal energy can be
used to recrystallize and grow grains during later heat treatment.
In summary, the higher the cooling rate during additive manufacture
the lower the temperature needed to recrystallize the metal and
fall within the grain growth regime during heat treatment. As will
be discussed in greater detail below, this thermal history
relationship means that in embodiments in which the heating/cooling
is set at a faster rate, the post-processing heat treatment will
grow grains within the deposited material at a lower temperature
allowing for a longer time for grain growth. In some embodiments
the thermal history of the pre-heat treated element may be varied
by altering the nature of the laser emission (e.g., by altering the
laser spot size, raster speed, power, movement or layer thickness)
such that faster or slower cooling of the element is achieved.
[0082] Embodiments incorporating such thermal history control allow
for the engineering of grain growth to obtain either fine or coarse
grain structure, wherein materials formed with a coarse grain
structure have fewer grain boundaries making it easier to magnetize
(e.g., lower coeercivity), and materials formed with a fine grain
structure have more grain boundaries making it harder to magnetize
(e.g., higher coeercivity). At the same time, smaller grain sizes
generally leads to stronger mechanical properties because the small
grain leads to a greater number of grain boundaries that in turn
operate within the material to prevent dislocation motion.
Accordingly, by varying the thermal history during deposition,
according to embodiments, it is possible to control the grain of
the soft magnetic material (e.g., creating smaller, larger, or
mixed grains within elements) which, in turn allows for the control
of the magnetic properties (e.g., coercivity, permeability, etc.)
and the mechanical properties (e.g., hardness, strength, etc.) of
an as-formed element.
[0083] Accordingly, embodiments employing microstructure control
allow for the properties of the soft magnetic element to be
engineered at the design stage and optimized for a specific
application. For example, magnetic texture may be leveraged to
improve the performance of magnetically shielded hall thrusters by
considering such texture in the design of the magnetic circuit.
Furthermore, the additive manufacturing process allows unique
processing considerations, such as radial deposition, that could
enable radial texturing of the magnetic properties potentially
useful for the magnetic circuit and otherwise not achievable using
traditional manufacturing processes.
[0084] Moreover, for larger elements (such as, for example, Hall
thrusters) that conventionally utilize bolted interfaces to join
different metal alloys, such as vacuum induction melted, vacuum arc
remelted (VIMVAR) iron and Fe--Co--V, embodiments implementing
variable grain growth additive manufacturing allow for the
fabrication of monolithic structures with compositional and
microstructural gradients to control property distribution through
the transition, eliminating the need for bolted joints, and
improving system performance and reliability. Furthermore,
compositional gradients between different materials (e.g., soft
magnetic alloys (e.g., Fe--Co/Fe--Co--V) and weldable ferrous metal
alloys (e.g. stainless steel)) can also facilitate reliable joining
of annealed soft magnetic alloy materials without degradation of
the magnetic properties from welding.
[0085] In summary, embodiments provide systems and methods allowing
for the use of various additive manufacturing processes to enable
the near-net fabrication of soft magnetic metal alloys in a variety
of applications including high-performance motors, high-efficiency
generators, solenoids, sensors, and the enabling magnetic shielding
for high-reliability, high-performance Hall thrusters. Embodiments
allow for reduced cost and lead-time for the production of
components. In addition, embodiments also allow for the spatial
control of the developing microstructure to, spatially, optimize
magnetic and mechanical properties for enhanced system performance.
The ability of embodiments to use a variety of fabrication
techniques further allows for the formation of any complex
geometry. For example, the use of a powder bed fusion (PBF)
process, in accordance with various embodiments, enables
fabrication of more complex geometries while still allowing
microstructural control of the printed, annealed components.
[0086] Accordingly, embodiments allow for the end-to-end processing
of selected soft magnetic alloy powders through custom
magnetic/mechanical parameters in an additive deposition system to
produce near fully dense (>99.5%) near-net shape soft magnetic
components, and post-processing those components to target
state-of-the-art magnetic properties or trading some magnetic
performance for mechanical properties such as hardness and
strength.
EXEMPLARY EMBODIMENTS
[0087] The following discussion sets forth embodiments where the
soft magnetic alloy additive manufacturing processes according to
embodiments may find particular application. It will be understood
that these embodiments are provided only for exemplary purposed and
are not meant to be limiting.
Example 1: Studies of Microstructure Properties of Soft Magnetic
Elements
[0088] Processing parameters according to embodiments were studied
to determine how to implement dense Hiperco.RTM. 50 test rings for
DC magnetic testing. Magnetic testing of these rings in as printed
and annealed conditions are summarized in FIGS. 4 to 7.
[0089] First, FIGS. 4a and 4b provide a set of SEMs demonstrating
the effect on the grain size of an as-printed element formed
according to embodiments from a Hiperco.RTM. 50 metal alloy. As
shown, the grain size enlargens under the effect of the annealing
process. This has a number of effects on both the magnetic and
mechanical properties of the element. Specifically, as shown in
FIG. 5, the magnetic flux density of the element increases, and as
shown in FIG. 6, the hardness of the element decreases. Finally,
FIGS. 7a and 7b provide graphs and tables summarizing the effect of
varying annealing conditions on the magnetic properties of
embodiments of magnetic elements. As shown, the magnetic properties
are sensitive to the specific conditions, and show significantly
degraded magnetic properties when minimum heat-treatment
parameters, according to embodiments are not met.
[0090] The magnetic properties against state of the art machined
elements are summarized in Table 3, and demonstrate that additively
manufactures Hiperco.RTM. 50 can be fabricated according to
embodiments and demonstrate magnetic properties equal to or
exceeding the current state-of-the-art (SOA) Hiperco.RTM. 50
magnetic elements.
TABLE-US-00003 TABLE 3 Magnetic/Mechanical Properties in
As-Printed/Annealed States Property As-Printed Annealed SOA*
B.sub.r (T) 1.21 1.51 Hc (Oe) 6.819 1.099 1.131-2.513.sup.a
.mu..sub.max 940 7597 4000-8000.sup.b Max Test B (T) 2.30 2.39
2.3.sup.b *State-of-the-Art: .sup.aR. H. Yu, et a., IEEE Trans.
Magn., 36 (2000), 3388-3393 .sup.bR. S. Sundar, et al., Int. Mat.
Rev. (2005)
[0091] Note, that tests on the magnetic properties of soft magnetic
elements formed using additive manufacturing methods according to
embodiments show that there is very little anisotropy. This can be
contrasted with conventional wrought materials that show
significant anisotropy in material properties. This anisotropy can
further complicate the subsequent manufacturing of and be
detrimental to the performance of the final part. Accordingly, the
relative lack of anisotropy in elements made in accordance with
embodiments provides another advantage to forming complex elements
having soft magnetic properties.
[0092] In summary, these results demonstrate that near net shape
magnetic elements can be formed from a Hiperco.RTM. 50 metal alloy
using methods according to embodiments. These elements have
densities approaching .about.99% using specific energies on the
order of 46 J/mm.sup.2. What is more, elements undergoing the full
manufacturing process: additive manufacturing and combined heat
treatment achieve magnetic properties comparable or exceeding
state-of-the-art wrought magnetic elements. These results also
demonstrate the unexpected sensitivity of these elements to
variations in heat treatments, and the importance of maintaining
strict control in accordance with embodiments of the process to
ensure the magnetic suitability of additively manufactured magnetic
elements.
Example 2: Studies of Additive Manufacturing Gradient Material
Elements
[0093] As previously discussed, while the magnetic properties of
soft magnetic alloys, such as Fe--Co alloys for example, are among
the highest of any known material, their strength, ductility and
fusion weldability are very poor. With respect to fusion
weldability specifically, Fe--Co alloys are subject to low
elongation failure, no necking and brittle microscopic features.
While there has been some recent success with solid state welding,
such as, for example, friction inertia welding, the final fracture
is brittle in nature and the Fe--Co is prone to fracture away from
the weld. Various embodiments are directed to the use of additive
manufacturing techniques to directly build soft magnetic elements
in combination with other materials and elements formed of
different materials thus removing the need for welds or other types
of joints. Embodiments may employ gradient additive manufacturing
to alter the composition of the alloys across the part to allow
fabrication of monolithic structures with spatially variant
properties without the need for mechanical fasteners. Such gradient
techniques, according to embodiments, can enable new design
possibilities and improve performance in severe mechanical
environments. Suitable gradient materials may include, but are not
limited to austenitic stainless steel alloys and ferrous metal
alloys (e.g., Invar, Inconel, Maraging SS, etc.) FIGS. 8a to 8e
demonstrate elements and the properties of elements formed using
gradient techniques in accordance with embodiments.
[0094] First, FIG. 8a shows an image of a gradient rod formed in
accordance with embodiments. The rod has a 1/2 inch diameter and is
graded over a .about.3 mm thick region over a series of .about.13
layers. (It will be understood that these gradient length and layer
numbers are exemplary and any desired length scale and number of
gradient layers may be used to accomplish a specific material
goal.) The exemplary rod transitions between SS316 and Hiperco.RTM.
50, where in this instance the SS316 was deposited first and
transitioned to Hiperco.RTM. 50, although any order may be used. In
addition, although this is a binary transition, it will be
understood that multiple transitions between two or more materials
may be undertaken. FIG. 8a shows that the gradient part may be
formed according to embodiments in a unitary piece without any
mechanical interconnection and without defect (e.g., crack
free).
[0095] FIG. 8b shows the formed rod element in an SEM image
revealing the formation of columnar grains nearer to the transition
between the materials (e.g., transition to BCC region). In turn,
FIG. 8C shows an expanded view of one region showing the relatively
sharp transition between FCC and BCC crystal structures. These SEMS
demonstrate that gradient elements can be formed that have
continuous transitions from one material to another even where the
materials have radically different crystal structures. FIG. 8d
shows the microhardness value of the gradient rod element in a data
plot where the left hand is the Hiperco.RTM. 50 alloy and
transitions to SS316 on the right. As shown, hardness peaks in the
transition zone between the ends. FIG. 8e demonstrates micrograph
images showing that there are no cracks in the material and minimal
pores indicating that a high quality, defect free gradient elements
can be formed according to embodiments. The availability of such a
process provides a method for manufacturing complex magnetic
elements with diverse materials integrally interconnected therewith
without the need for welding or other mechanical interconnection.
What is more, the ability to have a pure Fe--Co material means that
the element should behave magnetically identically to previously
described pure heat treated elements.
Example 3: Studies Implementing Hall Thruster Elements
[0096] Hall thrusters are an important element in the suite of
propulsion technologies being contemplated for use in many space
missions. In some embodiments, additive manufacturing methods as
described are used to rapidly manufacture magnetically shielded
Hall thrusters elements. Specifically, in the following studies
manufacturing parameters were developed to fabricate dense
Hiperco.RTM. 50 test rings for DC magnetic testing in accordance
with embodiments. Magnetic testing of these rings in as-printed and
annealed condition demonstrate that additively manufactured
Hiperco.RTM. 50 elements could be fabricated with magnetic
properties equal to or exceeding the current state-of-the-art
(SOA).
[0097] First, as shown in FIG. 9a, manufacturing models were
produced from a base MASMI Hall thruster design. To ensure the
thruster design was suitable for manufacture by additive processes
according to embodiments, models accounted for additional material
to allow for finish machining and un-supported overhang areas of
the design. Fillets were also added to the base for attachment to
the build plate. Using such models it is possible to varying the
manufacturing parameters (e.g., scan speeds and so on) to improve
on the quality of difficult overhang areas of the design.
[0098] As previously discussed, printing process parameters can
effect recrystallization and grain growth kinetics, and larger
grain sizes can be used to ensure improved magnetic properties,
while smaller grain sizes can lead to improved mechanical
properties within as-built magnetic elements. Although not to be
bound by theory, a possible mechanism relates to residual stress,
where smaller melt pools (e.g., more layers and melt pool
boundaries) can result in higher residual stress. Regardless of the
mechanism, as shown in the micrographs depicted in FIG. 9b,
different manufacturing parameters (e.g., laser spot size, energy
flux, etc.) can be applied locally within a thruster element to
induce desire mechanical or magnetic properties
[0099] A similar result can be seen in the SEM images of FIG. 9c,
which show that significant grain size difference can be seen
between low and high power builds according to embodiments.
Moreover as shown in FIG. 9d, VSM data indicates no evidence of
magnetic anisotropy in printed materials. This confirms the
magnetic advantages of additive manufacturing processes according
to embodiments.
[0100] FIG. 9e shows an image of a Hiperco.RTM. 50 magnetic test
ring and metallography blanks printed with varied energy density in
accordance with embodiments. In these embodiments, the layer
thickness and laser spot were held constant, and scan speed and
laser power were varied in the following configurations:
High/Low/High and Low/High/Low. These test coupons demonstrate the
possibility of controlling magnetic & mechanical properties
within a part, as shown by the difference in deposited structures
provided in FIG. 9f. A more detailed view of the grain size
variance between the different sample elements is provided in FIG.
9g. These views clearly show the possibility of using variance in
additive manufacturing parameters, according to embodiments, to
enable different grain sizes and thus concomitant variance in the
magnetic and mechanical properties of the thruster element.
[0101] As previously discussed, binary FeCo is reported to have
higher saturation and permeability with lower coercivity as
compared to Hiperco.RTM. 50 (Fe-49Co-2V). In addition, it has
higher electrical and thermal conductivities. However, FeCo has
poor mechanical properties which make it difficult to form and
machine. Hence, Hiperco.RTM. 50 is the state of art alloy for
forming magnetic elements in Hall thrusters. FIGS. 9h, 9i, and 9j
provide the results of printing experiments showing that
embodiments of additive manufacturing may enable use of higher
magnetic performance FeCo for thruster applications, and also allow
for variance of mechanical and magnetic properties by changing
manufacturing parameters, as discussed above.
[0102] FIGS. 9k and 9l also show images of another possible
parameter that can be used to alter the properties of the
manufactured element, namely the porosity of the powder feedstock.
As shown in the images the completed powder cross-sectional
analysis on both feedstock materials shows an observed correlation
between porosity in the manufactured part and the powder
feedstock.
[0103] FIG. 9m provides magnetic flux density measurements of the
Hiperco50 as-printed rings. As shown, all of the rings show
magnetic properties that are at least as good as state-of-art
wrought magnetic ring elements. Although not shown, given the
superior magnetic properties of the binary Fe--Co alloys, it is
anticipated that those rings will show magnetic properties
similarly superior to the Hiperco.RTM. 50 rings.
[0104] Taken together, the tests demonstrate the ability to
additively manufacture magnetically shielded Hall thrusters using
embodiments. This ability enables potential performance
improvements through the ability to control the microstructure (and
thus material properties), metallurgical gradients and integrated
thermal management to improve system performance of larger
thrusters, and near-net shape fabrication to reduce cost and
schedule. These benefits can be mission enabling for the types of
thrusters needed to explore and study the Solar System and beyond.
By developing a thorough understanding of the
process-structure-property-performance relationship for additive
manufacturing of soft magnetic alloys and gradients future thruster
designs can consider spatially controllable properties. Embodiments
can also be applied to other areas where soft magnetic alloys could
be beneficial, such as high efficiency motors.
Example 3: Studies of Energy Density Variance
[0105] In another exemplary study, the ability to vary the
properties of elements based on varying the deposition energy in
accordance with embodiments was studied. In these studies three 1/2
inch square rod coupons were deposited in accordance with
embodiments with the energy density gradient patterns summarized in
Table 4, below. In these samples the first segment (i.e., closest
to the build plate was 20 layers thick and each of the second and
third segments 15 layers thick. It will be understood that these
thickness are only exemplary and many variations may be made.
TABLE-US-00004 TABLE 4 Energy Density Variance for Samples Sample #
Segment 1 Segment 2 Segment 3 1 High Low High 2 High Low High 3 Low
High Low
[0106] FIG. 10a provides an optical micrograph of a cross-section
of Sample 1 showing the melt pool contrast across the sample as the
energy density is altered from a high energy density deposition
(.about.58 J/mm.sup.3 (high) to 31 J/mm.sup.3 (low) and back).
FIGS. 10b and 10c provide close-up micrographs of the high (FIG.
10b) and low (FIG. 10c) melt pools. These images show that
embodiments provide an ability to create continuous gradient
elements by varying the energy density of deposition.
DOCTRINE OF EQUIVALENTS
[0107] Although the present invention has been described in certain
specific aspects, many additional modifications and variations
would be apparent to those skilled in the art. It is therefore to
be understood that the present invention may be practiced otherwise
than specifically described. Thus, embodiments of the present
invention should be considered in all respects as illustrative and
not restrictive.
* * * * *